1. Field of the Invention
The present invention relates to electrical resistance temperature sensors or detectors (RTDs), and it relates more particuarly to a long, slender, continuous RTD capable of sensing continuously over an extended field.
2. Description of the Prior Art
Both thermocouples and resistance temperature sensors (RTDs) are in widespread use for sensing temperature and providing an electrical output representative of the temperature sensed. Thermocouples, by their nature, are point sensors because they thermoelectrically produce an e.m.f. at a specific junction between two different metals. RTDs employ a wire sensing element which has a resistance that varies according to temperature. Present RTDs are designed to concentrate the electrical resistance at a small point or in the smallest possible volume, with miniaturization being a principal feature so that RTDs are, like thermocouples, essentially point sensors. Because of this point sensing feature of both thermocouples and RTDs, wherever an extended field is to be interrogated with the use of either thermocouples or RTDs, it has heretofore been necessary to distribute a multiplicity of thermocouples or RTDs at selected points in the field.
No matter how many point sensing thermocouples or RTDs are distributed in a field, they still are unable to provide an accurate analog representation of the information to be determined from the field, because they are still only sensing specific points in the field, and determining the best points to interrogate, installing the individual thermocouples or RTDs. Making the numerous required individual electrical connections to the point sensing thermocouples and RTDs as is currently done is cumbersome and expensive.
One type of sensing of an extended field by the point sensing thermocouples and RTDs is sensing of the average temperature of a field. It will be apparent that the larger such a thermal field is, and the more varied the temperatures across the thermal field, the more point sensing thermocouples or RTDs are required to obtain an average readout which is fairly representative of the average temperature of the field.
Another type of extended field interrogation currently made with thermocouples or RTDs involves gauging of the level or location of a phase change interface, such as the liquid level, or interface between liquid and gas, in a tank. Such liquid level gauging is currently accomplished with thermocouples and RTDs by arranging a series of spaced thermocouples or RTDs along the height of the tank, i.e., at vertically separated points in the field being interrogated. Where RTDs are employed for this purpose, a series of heated RTDs and companion reference RTDs are employed along the height of the tank. As liquid reaches each RTD point sensor, the sensor reports that it is wet when it becomes cooled by the higher thermal dispersion rate of the liquid than the air above it. However, the operator is unable with such point sensing to determine whether the liquid level is just at that particular point or at any level between that point and just below the next higher RTD sensing point. Further filling of the tank will result in discrete reports from the sequentially higher RTDs, while lowering of the liquid level in the tank will cause successive discrete reports from successively lower RTDs as they are uncovered from the liquid. For example, if ten sensing points are employed along the height of the tank each with an individual heated RTD sensor and a reference RTD sensor, the gauging can only be performed at ten individual stepped points, with total uncertainty of where the liquid level is between the points. The only way to reduce such uncertainty is to increase the number of sensing points, at correspondingly increased expense. A liquid level sensing system of this type is disclosed in applicant's U.S. Pat. No. 4,449,403, issued May 22, 1984 for "Guide Tube Inserted Liquid Level Sensor."
Accurate liquid level sensing is of critical importance in liquid storage vessels and reactor buildings and in the reactor vessels themselves of nuclear power plants to avoid accidents such as that at the Three Mile Island plant, where liquid level was misinterpreted. Where a series of vertically arrayed point sensing thermocouples or RTDs is employed to determine liquid level, not only is there a lack of desired accuracy by not knowing where the liquid level is between the sensing points, but liquid level changes may not be immediately sensed, since there can be a considerable change in liquid level prior to detection, so that a developing problem may not be immediately detected, and therefore mitigating action to suppress the problem would not be promptly taken by the operator.
Since each of the vertical sequence of thermocouples or RTDs in such present liquid level gauging systems requires its own separate electrical connections to the detection circuitry, the required large number of electrical joints or splices results in undesirably low reliability, which could be dangerous in the nuclear power plant environment. As an illustration of how serious this problem can be, applicant is familiar with one point-sensing RTD system for gauging water level in a nuclear reactor building which has as many as fifty RTD sensors arrayed over a vertical height of approximately sixty feet.
RTDs are generally preferred for some purpose over thermocouples for most uses because they can be made much more sensitive, being able to provide an output signal many times greater than thermocouples. This is because RTDs operate with an external electrical power source, which can provide as high a voltage or current as is self-generated junction e.m.f., which inherently has a very low output voltage level as well as other inaccuracies. Nevertheless, for sensing some extended fields, such as the inside of a nuclear reactor vessel, access may be difficult, and best achieved by encasing a series of the sensors in a long, slender tubular probe. Such a probe can readily be inserted in an existing reactor vessel instrument guide tube. While it would be desirable to have RTDs so packaged because of their high output, and hence sensitivity, current state of the art RTDs are not suitable for such packaging, being much too bulky, and having a ceramic or glass insulator too brittle to allow them to be deformed as would be required for packaging them in such a long, slender tubular probe. Thermocouples, on the other hand, have been known to be packaged inside a metal casing as small as 0.010 inch in diameter, and a series of such encased thermocouples and the required electrical leads placed inside a tube and encased by drawing or swaging the tube down around the thermocouples and leads to produce a long, slender probe suitable for gaining access to constricted regions inside a nuclear reactor vessel. However, such thermocouple probes have serious disadvantages. First, the thermocouples are delicate and are easily subject to breakage during the manufacture of such probes or upon accidental impacting. Also, because of their inherent point sensing, the thermocouple-type probes necessarily have a step function output, rather than a continuous output, so liquid level cannot be accurately determined. Further, the electrical output of the thermocouples is so small that performance is grainy and resolution and accuracy are poor. Also, individual wire leads are required for each of the thermocouples, so that numerous wires must extend along the tube of the probe, which seriously limits how small the outside diameter of the tubular probe can be, and of course the larger the number of thermocouples placed along the probe in an attempt to increase resolution, the greater the number of leads. The large number of leads also seriously reduces the reliability of such thermocouple-type probes. Such thermocouple-type probes are also quite expensive to make, and it is even more expensive to provide leads, connections and electronic cooperating devices for thermocouple-type probes.
Another type of extended field which has been interrogated by a multiplicity of RTDs or thermocouples is a large duct having a nonuniform flow profile, where it is sought to obtain an average reading of the flow velocity in the duct. Such nonuniform flow distributions exist, for example, in air ducts where diameters are large and fittings such as tees, elbows, transitions, bends, section changes, louvers, dampers, and the like cause flow disturbances. Nonuniform flow distributions also typically occur in the input air ducts and combustion output ducts of fossil fuel power plants. In such cases, a multiplicity of point sensing RTD or thermocouple sensors are placed at what are considered to be strategic locations across the air or gas flow path, but only a rough approximation of the flow velocity can be obtained by use of such discrete, point sensing locations. Again, these individual RTD point sensors suffer from high costs of leads, connectors, and mating electronic devices that cooperate in interpreting the individual signals.